V/P Duo Prop. Would going from F5 to F6....

[carboncow+iboats]

Hey gang. Go easy on me from those with all the knowledge!

I restored a 1997 Pursuit Denali the last few years with a new long (big) block prob sitting at 310-330hp with a Volvo Penta duo setup, truly brought it back from the dead. I currently have F5 props that is came with...still in great shape.

One of the big challenges during my redo was getting new injectors figured out and last summer we got that sorted, so I"m confident they are flowing correct. As far as the fuel pump and pressure...haven't got that far yet to know if it's correct, I'll need a tech for that. But the FI system is all original, so ancient.

Boat specs. Approx. 5000-6000 lbs depending on which data I can find. I'd like to think I'm on the higher side since most of these were small blocks. I cruise at 3000 rpm and at mid season I'm getting about 27mph on a mild day. I can only get WOT to about 3800 and 36 mph. So pretty linear speed to RPM across that range. I seem to recall my crusaders in another boat were 4200-4400 RPM with velvet drives. The fact I have not gotten WOT the last 3 seasons since getting the new engine in (and new injectors last year) hasn't been an issue because I don't really wan to run much over 3000-3300RPM most of the time...

but. I'd love to have 2-3 more mph, maybe? Plus I idle at 600rpm and going no wake through our area is waaayyy to slow. Wondering if an F6 would help with...

1. idle speed? Even .5-.8 would be major improvement.
2. another 1,2 or 3 mph at my cruising speeds?
3. Get closer to an expected WOT, or is my understanding backwards...and I might have a power delivery issue with fuel or such?

If you look at the attached documents my speed compared to the tested 5.7L is pretty much spot on especially since my bottom can be a mess with growth half way through the season. But I was hoping the big block would get a little more speed...otherwise what is the benefit of a big block on this size boat at similar RPMs? It's got tons of torque for sure, feels like I could drag a baby elephant or two on tubes with ease. It hops on plane instantly with 5 adults on board.

1997 Pursuit Denali 2460 (25ft)
Weight is listed at 4200 Dry in one document and 5200 in another. I think I'm 5500+ with fuel, big block, etc.
Drive: DPS 1.95 radio
Engine: 7.4GiPLKDCE (new long block 3 summer ago and rebuilder said 330hp)
Props....
F5Front 3851465 7J
F5Rear 3851475 7J
See attach images of docs I have found on my approx. year but it seems 90% of this boat model were 5.7L engines and a lot of mercruisers.

 

[alldodge]

Your don't want more pitch you need less pitch to get the RPM higher. Need closer to 4800 RPM. The motor is lugging and will shorten it's life

It could be your new motor is not producing the HP it should. Check compression, should be close to 150 psi

Check the plugs and see what they look like, should be a cream color

 

[carboncow+iboats]

Your don't want more pitch you need less pitch to get the RPM higher. Need closer to 4800 RPM. The motor is lugging and will shorten it's life

It could be your new motor is not producing the HP it should. Check compression, should be close to 150 psi

Check the plugs and see what they look like, should be a cream color

Thanks.
New plugs went in this spring and we did check compression on 3 random cylinders (just checking) as we though we might have had one bad injector and wanted to check that one for any issues and checked two others for a baseline. All were 147-148.

My other theory was the actual cable run out might be limiting WOT and others mentioned fuel pressure.

 

[alldodge]

You said BB and at 310 to 330HP I take it's a 454 (Merc or VP) and MPI or TBI?

The spec for the boat with a 5.7 @ 280HP says it will do 48MPH and your only getting 36MPH, something is wrong. Either the motor is not producing the power or the drive ratio is to high

 

[Scott Danforth]

Weigh the boat
Check compression

You should be getting 4500 RPM easy with F5swith a healthy big block

Dry weight of 5200 would be wet wright close to 6400#

5200# dry weight
680# of fuel
25# of other fluids
100# of batteries
100# of anchors and lines
300+# of other gear
100# of canvas, etc

 

[jimmbo]

First make sure the Tach is accurate
Is the Spark advancing?
Is the Exhaust free, as in no blockage from relocated Flappers?
A Compression Test of all Cylinders

 

[QBhoy]

Hi. I’d be looking elsewhere for the issue or adjusting required. In all the years I’ve been about boats with a VP engine, drive and dp’s…(referring to an absolutely original and factory set up)…I’ve yet to know, witness or even hear of an example that didn’t display an almost perfect result, in terms of the rpms at full throttle being as close to spot on as it gets. Again…that’s as long as nothing has been messed with, in the lifetime of the boat.
Your issue has got to be elsewhere I’d wager. Trim, poor engine health or otherwise perhaps. My thoughts at least. Can’t remember if you said she was fuel injected with ecu or not…but it’s possible she may be in a gaurdian mode and restricted power